Farallon Islands mice plague threatens seabird colonies at 1,270 per hectare

Non-native house mice have reached densities of up to 1,270 per hectare on the Farallon Islands, 30 miles off San Francisco, according to research published in Ecosphere. Accidentally brought by 19th-century ships, the mice now disrupt the ecosystem of the islands, which host the largest seabird nesting colony in the contiguous United States with over 300,000 breeding birds from 13 species.

The huge mouse population creates an indirect predation problem. Each autumn numbers peak, then crash in winter, forcing migratory burrowing owls to switch from mice to the Ashy storm-petrel, a globally vulnerable seabird that nests on the islands. Point Blue Conservation Science, which has studied the islands since 1968, says the owls can cause significant adult deaths among storm-petrels, which lay only one egg per year and are slow to recover.

To break the cycle, the U.S. Fish and Wildlife Service has developed the South Farallon Islands Invasive House Mouse Eradication Project. The plan involves spreading rodenticide bait from helicopters in late autumn, when mouse numbers naturally decline and seabird numbers are lowest. The California Coastal Commission approved the plan in December 2021. Safety measures include temporarily relocating resident burrowing owls and using deterrents to keep Western Gulls away.